Many light-duty trucks and sport-utility vehicles are equipped with a transfer case for providing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, many transfer cases are equipped with a gear reduction unit that can be selectively shifted by the vehicle operator to establish high-range (i.e., direct drive) and low-range (i.e., reduced ratio drive) four-wheel drive modes. While layshaft-type gear reduction units are available, most transfer cases are now equipped with a planetary-type gear reduction unit due to improved efficiency and reduced size requirements. An example of such a planetary-type reduction unit installed in a four-wheel drive transfer case is shown in commonly-owned U.S. Pat. No. 4,770,280.
Due to inherent noise associated with spur gears, many planetary-type gear reduction units utilize helical gearing to provide quieter operation. However, a recognized design constraint associated with helical planetary gearsets is the opposing lateral thrust loads generated by the helix angle of the sun gear and ring gear must be contained. Moreover, it is desirable to allow the helical gearing to be self-centering for optimized sharing of the gear loads which may result in further improvements in durability and noise reduction. In the gear industry, it is known that herringbone gears can be used to address the thrust loading associated with conventional helical gearing. Herringbone gears, however, are difficult and costly to manufacture and, as such, have limited application to vehicular drivetrain products. As such, a need exists to develop planetary gear assemblies which advance the art and address the shortcomings of conventional planetary systems.